It's an interaction with TABPANE, because disabling JQueryPlugin allows TMCE to inherit the proper dimensions from the #topic textarea and it works fine.

The simple fix is TINYMCEPLUGIN_INIT and add width : "99%" there, or adjust the edit.natedit.tmpl to add the style attribute to the textarea.

However this only solves width and not height problems when the browser is resized.

I want to add some JS to edit.js along the lines of what TMCE's autoresize plugin does, but just so that it will keep TMCE's editor sized to fit the browser dimensions at all times.

This is a major usability issue and I have reports from some users that sometimes TMCE's editor window manages to cover save/cancel buttons at the bottom so that they cannot save their work (although I have been unable to replicate; these users are on IE6).